# Old Newbie with Some Simple Questions - 002

Discussion in 'General Electronics Chat' started by olnoob, Nov 19, 2013.

1. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
It's been a while since I was here. I posted a question several months ago and got great answers. Here are a few more that should be pretty easy for a lot of you guys. (Please remember that I am a total noob. )

1) What does one call a potentiometer/variable resistor that is also an on-off switch? They look like this:

and you can press the "handle" down to toggle the circuit open and closed. It's like two separate switches in one, in other words. On-off and variable resistance.

I ask because I need a few for a project. I want to buy them online and searching for an electronic component without knowing what it's called is no fun any time. lol.

2) I need these for a DC application. I need to be able to modify 5 to 15 volts DC to within 0.1 volts over a range of say 2 volts. (From 4.0 to 6.0 via 0.1 v steps, for instance. And from say 11.0 to 13.0 with the same degree of accuracy.) What "law" do I use to know what specs to shoot for?

Any help will be greatly appreciated.

2. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
Most important to design is the load current.

3. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0

I'll be using 3 of these.

1) 5V 15A
2) 12V 2A
3) -5V 1A
110 watts.

So #1 will need to go from 4V to 6V via 0.1V increments.

#2 will need to get me from 11 to 13, same accuracy.

#3 same as #1 except it's negatory. (And I don't EVEN understand what that means!)

4. ### #12 Expert

Nov 30, 2010
18,076
9,691
There is no such thing as a 15 amp potentiometer under \$100.
You are going to end up using the "potentiometer with switch" to control some transistors or regulator chips. This will require us to understand much more about your circuit. For instance, those voltages look like the power supply for a computer. What are you starting with and where are you trying to go?

5. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
I want to use a single power supply to power two devices: a computer and a video converter.

The power supply puts out 5V @ 15A, 12V @ 2A, and -5V @ 1A. It has a pot onboard. Using this single pot increases the voltage output on all three lines.

The video converter runs on 5V and works best at about 5.2V. The computer works best at 5.0V. I would like to crank up the supply voltage to at least 5.2V (which means about 12.2V and -5.1V on the other two lines). I want to be able to use a pot at the video converter to allow the 5.2V. I want to be able to use three other pots to feed the computer 5.0V, 12.0V, and -5V, respectively.

6. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
How about adjust the supply to get your 5.2 (can't believe it's that tightly spec'ed), then drop 1.4 volts off the 12.2 volts with a couple diodes.
And drop .6 or .7 from -5 with a single diode.

Careful diode selection may get closer.
Next guess is doing the same thing with pass transistors.

7. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
****. I'd no idea. Did some reading and guess things aren't what they seemed.

So you can't resist 5V @ 15 amps with a simple potentiometer. (Not for under \$BIG anyway.) The amount of energy that would have to dissipate at the pot would necessitate some serious refrigeration.

So how does the pot on the power supply work? It controls something that in turn modifies the voltage. What is that?

Edit: Yes, I can solder chips/IC's/whatnots onto PCB's. I just need an idea for the recipe.

8. ### #12 Expert

Nov 30, 2010
18,076
9,691
It's the voltage regulator chip.
Then the voltage regulator chip controls the big transistors.

9. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
A rheostat connection will work if the load is constant. You will lose regulation with different loads.
10 watts or so is practical.

Potentiometer would be pretty lossy. sp

Diodes have a fairly fixed voltage drop. Would make a simple solution to drop a few volts.

4 pass transistors and circuitry gets a little involved. Are you up for it?

File size:
93.5 KB
Views:
19
10. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
The regulation is most likely done ahead of the transformer. So all outputs track.

Schottky will get you close to the drops you need.

Cheap fix. Rob them out of some high current power supplies.

11. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
So you're saying that a simple rheostat will do the trick if its wattage rating is high enough?

If this is so then wouldn't I need a 75w rheostat for the 5V 15A line?

And a 25w one will suffice for the 12V 2A line?

And an even lesser (ie cheaper) one for the -5V 1A line?

------------------------------------------------------

I appreciate all the suggestions regarding diodes, but I have my heart set on being able to manually vary the voltages in this application.

12. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
If I had a diagram I'd be down. I ain't skeert!

If the unit would be smaller (and cheaper) than a 100w rheostat (which is larger than it is expensive, and it is that) then I wouldn't mind putting it together.

13. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
Bump!

OK. I ordered a 100w rheostat. How does one apply it to the circuit in order to alter the voltage (and not the amperage)?

This can be done, right?

14. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
Connect as a potentiometer.

Will lose regulation though.

Still like this idea!

15. ### olnoob Thread Starter New Member

Jun 26, 2013
10
0
Are you saying that the fluctuations of the load, perhaps natively insignificant, may become noticeable on the other side of the rheostat?

I would like to hear about the pass transistors.